Synthesis of Extracellular Pyrophosphate Increases in Vascular Smooth Muscle Cells During Phosphate-Induced Calcification

Objective Hydroxyapatite deposition on the medial layer of the aortic walls is the hallmark of vascular calcification and the most common complication in aging individuals and in patients with diabetes mellitus and those undergoing hemodialysis. Extracellular pyrophosphate is a potent physicochemical inhibitor of hydroxyapatite crystal formation. This study analyzed changes in extracellular pyrophosphate metabolism during the phosphate-induced calcification process. Approach and Results Phosphate-induced calcification of ex vivo-cultured aortic rings resulted in calcium accumulation after 7 days. This accumulation was enhanced when aortic walls were devitalized. BMP2 (bone morphogenic protein 2) expression was associated with calcium accumulation in cultured aortic rings, as well as in cultured vascular smooth muscle cells (VSMCs) and in calcitriol-induced calcification in rats. Hydroxyapatite dose dependently induced BMP2 overexpression in VSMCs. Moreover, TNAP (tissue nonspecific alkaline phosphatase) mRNA levels and activity were found to be downregulated in early phases and upregulated in later phases of calcification in all 3 models studied. eNPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) increased from early to later phases of calcification, whereas eNTPD1 (ectonucleoside triphosphate diphosphohydrolase 1) was downregulated during later phases. Synthesis of pyrophosphate in VSMCs increased significantly over time, in all 3 models studied. Because the rate of pyrophosphate hydrolysis was 10x slower than the rate of pyrophosphate synthesis, pyrophosphate synthesis is determined mainly by the ratio of eNPP1 to eNTPD1 activity. Hydroxyapatite also induces increments both in TNAP and eNPP1/eNTPD1 ratio in VSMCs. Conclusions Pyrophosphate synthesis increases in VSMCs during phosphate-induced calcification because of compensatory regulation of extracellular pyrophosphate metabolism.